Active electronic filters, particularly bandpass amplifiers, are well known in the art. They are usually constructed with at least one active device and several discrete passive resistive and reactive components. A dual-input differential amplifier, commonly referred to as an "operational amplifier," is an active device exhibiting a very high, open loop gain, over a wide bandwidth, a very high input impedance, and a very low output impedance. Widespread commercial availability of operational amplifiers and their nearly ideal characteristics make them the preferred active devices for use in active filters. Additionally, operational amplifiers are readily suited to integrated circuit fabrication. To take advantage of the economy, compactness and versatility of integrated circuits, it is desirable to design amplifier circuits which are suitable for integrated circuit fabrication. Amplifier circuits having only resistive passive components are particularly suitable because they may be made as monolithic devices capable of functioning without external passive impedance components.
Recent proposals for integrated active filter circuits having operational amplifiers include the class of filters called "active R" filters. These filters are constructed with only resistive passive components. Amplifier circuits having only resistive passive components provide two significant advantages. First, resistances can be made to exhibit greater stability than any of the other passive components. Second, resistance components may be made easily adjustable, thereby enabling a circuit to be tuned over a band of frequencies without significantly effecting stability of the circuit. One proposed design relies upon the existence of a resistive-capacitive equivalent in the transfer function of a differential input amplifier to form a bandpass filter with only two passive components, both resistances, and two operational amplifiers. The performance of that particular design provides only minimal improvement over an equivalent circuit constructed with transistors because it relies upon a change of gain with frequency to provide filtering and lacks both the tunability and sharpness of conventional inductive-capacitive tuned filter circuits.